1. Field of Invention
The present invention relates to semiconductor devices, and particularly to a semiconductor device which is provided with MOS transistors formed on a semiconductor layer on an insulating layer and which can prevent supporting substrate stray effects.
2. Description of Related Art
A silicon-on-insulator (SOI) technology, which includes forming a semiconductor layer of single-crystal silicon on an insulator and forming semiconductor devices such as transistor devices on the semiconductor layer, has advantages, such as high-speed device operation, reduced electrical power consumption, and high integration density, and may be applied to electro-optical devices, such as liquid crystal devices.
In typical bulk semiconductor components, a channel region of a MOS transistor is maintained at a predetermined potential by an underlying supporting substrate. Thus, a parasitic bipolar effect due to a change in potential of the channel region does not cause deterioration of electrical characteristics, such as breakdown voltage of the device.
In a MOS transistor having an SOI structure, however, a channel bottom section is completely separated by an underlying insulating film. Thus, the channel region cannot be maintained at a predetermined potential, and is in an electrically straying state. In such a state, excessive carriers are accumulated in the channel bottom section in which the excessive carriers are generated by impact ionization due to collision of carriers accelerated in an electric field in the vicinity of a drain region with crystal lattices. When the channel potential increases by the accumulation of the excessive carriers in the channel bottom section, the NPN structure (in the case of N-channel type) of the source/channel/drain operates as an apparent bipolar device and yields an extraordinary current which causes deterioration of the breakdown voltage between the source and the drain, and thus, deterioration of electrical characteristics of the device. A series of phenomena caused by an electrical straying state of the channel region is called a supporting substrate stray effect.
A conventional method for suppressing the supporting substrate stray effect is providing a body contact region which is electrically connected to the channel region via a predetermined path in order to extract the excessive carriers in the channel region from the body contact region.
A semiconductor device including MOS transistors of an SOI structure having such a body contact region is disclosed in Japanese Unexamined Patent Application Publication No. 9-246562 (hereinafter Citation 1).